1. Field of the Invention
The present invention generally relates to a sheet processing device, an image forming system, and a sheet processing method, and, more particularly, to a sheet processing device and an image forming system with a reduced space portion for a stacking unit, on which sheets are to be stacked when stapling is performed, and a sheet processing method to be performed in a sheet processing device and an image forming system.
2. Description of the Related Art
In an electrographic image forming apparatus, there is known a sheet finishing device capable of stapling, with a staple unit, at one or more predetermined positions on the accumulated sheets each of which are ejected from the body of the image forming apparatus and stacked temporarily on a so-called “staple tray”. Conventionally, this type of sheet finishing device includes or a device capable of changing a direction of staple needle by rotating horizontally the staple unit in order to staple at one corner or one edge of bundle of sheets disposed on the staple tray inside of the device, a device capable of moving the staple unit along one edge of the bundle of sheets in order to staple plurality positions on the one edge, and the like.
For example, Japanese Patent No. 3399667 discloses a sheet processing device that requires no staple tray for stapling treatment, in order to downsize the device.
Japanese Patent No. 3399667 intends to simplify and downsize the mechanism of the sheet processing device (sheet finishing device) having a staple function. And, for the purpose of continuous stapling operations, the device is provided with a sheet eject tray that moves up and down depending on the stack amount of the sheets thereon, an auxiliary tray disposed between the sheet eject tray and a sheet eject roller to eject the sheet onto the sheet eject tray and movable with respect to the device body in parallel to the sheet conveying direction between a position covering above a portion near the base of the sheet eject tray and a position retrieved from the above position, and a staple unit disposed aside of the auxiliary tray and capable of staple the bundle of sheets while reciprocating in a direction orthogonal to the sheet width direction.
However, the sheet processing device (sheet finishing device) having stapling function for plurality positions needs (i) to maintain a space allowing the movement of the staple unit in the case of two position stapling which requires the movement of the staple unit, and (ii) to maintain a wider space on the tray in order to ensure the accurate function of align roller or jogger fences to align the sheets for the purpose of ensuring the alignment quality of the bundle of sheets stacked on the staple tray. Therefore, the conventional sheet processing device maintains the space allowing the movement of the staple unit and the space for the layout of rollers and fences on the tray. Thereby, it is difficult to downsize this type of conventional sheet processing device. Specifically, it is very difficult to maintain these spaces in the limited space of the inner-body sheet processing device, for example.
On the other hand, the device disclosed by Japanese Patent No. 3399667 realizes the downsizing of the device while maintaining the staple function. However, the device is not configured to allow the movement of the staple unit inward the device to realize the two position stapling. Furthermore, the staple tray is an auxiliary tray capable of projecting toward and retrieving from the sheet eject tray. Thereby, if the area of the staple tray increases, the staple tray covers a space above the sheet eject tray. Specifically, in the case of the inner-body sheet eject tray, the movable range thereof in the vertical direction is limited. As a result, a maximum stack amount on the sheet eject tray reduces in the case of the sheet processing unit employing the fixed tray. On the other hand, if the area of the staple tray reduces, it becomes difficult to satisfy the accuracy in the alignment for stapling, since the alignment roller becomes difficult to be disposed at the optimum position and since the alignment in the width direction by jogger fences becomes almost impossible.
There is a need to present a sheet processing device capable of maintaining a space for the movement of the staple unit, and capable of maintaining a space for the layout on the staple tray to satisfy the alignment accuracy for stapling.